River Dynamics and Channel Morphology

Pressure and Sediment Transport

Overview of the field site with river water inflow and a reservoir.
Three experiments: free-flowing conditions, river conditions, and dam removal.
Sediment transport is the central theme.
Relate to previous lectures on alluvial terraces and delta formation.
Delta formed in the reservoir partly eroded after dam removal, leaving remnants.

Dam Removal

Dam removal video to illustrate principles. Focus on channel shapes.
Water velocity is critical for sediment transport.
Deltas form in reservoirs, not river mouths.
Lowering reservoir levels after dam removal causes delta erosion and sediment transport downstream.
Yellow Wah dam removal was slow, with gradual chipping and flow diversion.
Video shows a rapid dam breach using explosives in a rural area near the Columbia River.
Rapid reservoir drawdown observed in real-time.

Sediment Dynamics

Super sediment-plain water moves quickly with high suspended load.
Delta surface erosion as the channel cuts through it.
Terraces remain above the new channel level.
Sediment trapped over the dam's lifetime is significant.
The new channel incises and erodes sediment.
Trees drowned when the reservoir was constructed become visible.
Winter floods can mobilize loose sediment downstream.
Revegetation stabilizes terraces over time, redistributing sediment.

Short-Term vs. Long-Term Impacts

In the first year after dam removal, sediment discharge can be very high.
High sediment levels act like sandpaper in the water, negatively impacting fish.
Long-term benefits include removing barriers to fish passage and reintroducing sediment, improving habitat complexity.

Environmental Impact and Considerations

Environmental impact statements consider services provided by the dam (hydropower, recreation, navigation).
Evaluates costs and benefits of dam removal.
Each system is unique; there's no universal decision.
Sediment accumulation varies; contamination is a concern.
Downstream impacts on humans and loss of services are considered.

Case Studies

Elwha and Klamath River dams had negligible hydropower production.
The owner decided benefits did not warrant continued operation.
Framingtouli Dam probably won't be removed due to its benefits.
Fish quickly reorient to the new habitat post-dam removal.
Reproductive success improves over a few years.

River Modification

Over 99% of rivers and streams in the US are altered.
Dam removal is an unusual intervention.
Prioritization aims to balance benefits by reducing generation capacity and considering tribal treaty rights.
Dam removal is gaining traction but involves relatively small structures.
Shifting societal views on the purpose of rivers affect dam removal decisions.
Main stem Columbia River dams are unlikely to be removed soon.
White Salmon River dam removal was a win for fish habitat at a low cost.

Trade-offs of Dams

Dams are impressive engineering feats with societal benefits.
They also have costs, requiring prioritization.
Cheap electricity and food production are subsidized by dams.
Controversy arises as dams require relicensing with new fish passage requirements.
Economic costs of hydropower vs. fish passage influence decisions.
Upgrades are expensive; efficacy varies.
Philosophical questions arise regarding the ethical implications of dams on rivers.
Scientific questions persist regarding fish health and passage.

Enforcement and Regulation

Federal and state authorities enforce regulations on projects using federal money.
Environmental laws in the 1960s and 1970s allowed citizens to sue the federal government to ensure protection of resources.
Prior to 1970, citizens had limited direct checks on federal agencies.
Dams were a major catalyst for the environmental movement.

Dam construction

Rivers are diverted using coffer dams.
Construction begins in the diverted channel.
Hoover Dam construction involved untested techniques and risks.
New Deal engineering projects aimed to create jobs, but many people died.
Dam failures occur; Keaton Jamfel on YouTube provides examples.
Reservoirs can depress the earth's surface.

Channel types

Flowing water erodes sediment, with faster water eroding more and carrying larger particles.
As water slows, particles deposit, influencing channel shapes and water movement.
Three channel types are explored: straight, meandering, and braided, influenced by velocity changes.
A single river can exhibit multiple channel types depending on location and constraints.
Canoeists in meandering channels are influenced by channel features.

Straight Channels

In straight channels, water flows in a straight line with spatial velocity variations.
Faster water is in the center, slower on the sides.
Friction occurs at banks and the riverbed, creating slower water zones along the wetted perimeter.
The fastest water is just below the surface in the deepest part of the channel, away from friction sources.
Faster water in the center erodes material, maintaining the channel.
Deposition is less likely in the fastest-moving part of the channel.
The thalweg is the deepest part of the channel with the fastest-moving water and least deposition.
Straight channels are rare in nature, requiring stability or human intervention.
Natural straight channels are caused by strong geologic controls.
Human-straightened channels have levees to confine the channel.

Meandering Channels

Disturbances in natural systems alter flow. An obstruction, like a fallen tree, causes water to bend.
Water is forced to bend around the tree, it erodes the opposite bank.
This deepens and widens the channel section. Faster water erodes more, creating a positive feedback loop.
Water slows at the tree, depositing sediment.
A bar forms, constricting the area where water can flow.
The channel meanders, creating bends as water swings to the curve's outside.
Faster water erodes; slower water deposits sediment.
Meandering channels require disturbance and space to move.
The thalweg is not in the center; it's on the eroding side.
Fastest water promotes erosion; deposition occurs elsewhere.
Straight channels become unstable without human maintenance.
Human intervention maintains channels with boulders, concrete walls, and levees.
Thalweg is in the center; slow water is along banks.
{Channel dredging} deepens channels for larger ships, particularly on the Columbia River.
Dams on the Columbia help navigation by creating calmer river sections for barges.

Erosional and Depositional Features

Meandering systems create erosional and depositional features or mini landforms formed by erosion or deposition.
Glaciers also produce erosional and depositional features.
Meandering is when the channel length is 50% greater than a straight line between flow points.
Thalweg moves back and forth, water hugs the bank, deepening the channel, which creates a cut bank or an erosional feature.
River sediment deposits to create a point bar. On the side of slower movement, a zone likely to deposit sediment.
Pairs of point bars and cut banks alternate downstream.
Erosion cuts through riverbanks, abandoning old meanders, resulting in oxbow lakes and meander scars.
Oxbow Lakes are old channel portions being filled with water. Meander scars are depressions in the landscape from old rivers not filled with water.
Water depth is shown based on lighter and darker blues, with darker blue for deep water and shallow water for light blue.
A nice profile that shows a fool wage on a straight section, our fool wage in the middle.
The length
of
one
shaped
meander
is
six
times
the
width
of
the
channel.
Oxbow lakes form when a river cuts through a meander neck, abandoning the old loop.
Features of include: meander scars, oxbow legs,
point bars, cut banks
A uniform surface never occurs and human modification is often messily superimposed.
Dams help control floods, which is a problem if wanted to go up next river.
High water flows and overflows the banks. Thin film slows it down deposits sediments. River and streams result in rich soils from floods.
There are natural levees and human levees to constrain the channels so as to prevent flooding.
In order for Oxbow Lakes to permanently disappear water is required to evaporate.
Rivers political boundaries can cause a problem for state properties and state lines when rivers change or disappear.
Yazoo Tributaries are a symptom of flooding collecting water and natural rivers being a problem.
Rivers move across the landscape over time, which is important to note.

Braided Channels

This channel is an overloaded system with deposition always occurring caused by plentiful sediment or slow moving water.
It is common in the headwaters of glacially fed rivers due to glaciers eroding material off the landscape.
It is seen below glacial peaks and high in glacial milk water, leading to sediment.
Temporal patterns occur caused by snow and ice melt which causes run off resulting in discharge.
Flowing water is trying to pick the most efficient path through constant deposition. It flows, it slows, it deposits.
The water is going to channel the next easiest way. A rate exists as water picks that channel immediately and as discharged increases or increases the channel width might increase.
The channel type is common in glacial systems that are really minimal.
The gradient of the channel is steepness from one point or another and where areas are flat, water moves slower.
Desert streams are good examples of channel gradient and very variable in precipitation.
There are little islands or bars that vegetated in sediment where vegetation causes stabilization while disturbance prevents stabilizing vegetation.
A channel width exists is and they're common and areas with plentiful mobile sediment located near downstream glasses